FIG. 9 is a cross-sectional view of a microelectromechanical device (referred to as “electronic device” hereinafter) 1 of related art. The electronic device 1 is constituted by mounting an electronic component 5, which includes a microelectromechanical system (referred to as “MEMS” hereinafter) 3 and an electrode 4 disposed on a semiconductor substrate 2, onto a first substrate 6 also known as a covering substrate while sealing the MEMS 3. The electronic device 1 can achieve size reduction since the MEMS 3 is sealed with two substrate while being electrically connected to a wire 7 penetrating the first substrate 6 (for example, see Japanese Unexamined Patent Application Publication Nos. 2002-43463 and 2004-296724).
FIG. 10 is a cross-sectional view showing the state in which the electronic device 1 is mounted on a printed board 8. The electronic device 1 is mounted on the printed board 8 by bringing a main surface of the first substrate 6 remote from the semiconductor substrate 2 to face the printed board 8 also known as a circuit board. When the electronic device 1 is mounted on the printed board 8, thermal stresses are generated due to a large difference in thermal expansion coefficient between the printed board 8 and each of the substrates 2 and 6 of the electronic device 1, and strains are generated in the semiconductor substrate 2. If strains occur in the semiconductor substrate 2, the MEMS 3 is likely to exhibit degraded functions. The operation of the MEMS 3 is severely affected even by slight strains.
FIG. 11 is a cross-sectional view showing another state in which the electronic device 1 is mounted on the printed board 8. In order to reduce strains in the semiconductor substrate 2, a mount structure is employed as shown in FIG. 11, in which a second substrate 9, also known as “interposer” or the like, is interposed between the printed board 8 and the electronic device 1. A substrate that has a thermal expansion coefficient between that of the substrates 2 and 6 of the electronic device 1 and that of the printed board 8 is used as the second substrate 9, and this moderates the thermal stresses generated in the semiconductor substrate 2.
According to the structure shown in FIG. 11 including the second substrate 9, the electronic device 1 is arranged in such a way that a main surface of the first substrate 6 remote from the semiconductor substrate 2 faces the second substrate 9. Thus, in order to electrically connect the MEMS 3 to conductor line (not shown) formed on the printed board 8, wires 7 penetrating the first substrate 6 in the thickness direction must be formed. This renders the structure of the electronic device 1 more complex, which is a problem.
Although the strains in the semiconductor substrate 2 can be reduced to a certain degree by interposing the second substrate 9 between the electronic device 1 and the printed board 8, the degree of suppression is not sufficient. This is due to the following reason. The thermal expansion coefficient decreases in the order of the printed board 8, the second substrate 9, the first substrate 6, and the semiconductor substrate 2, that is, the thermal expansion coefficient decreases with an increase in distance from the printed board 8. Thus, in all of the substrates 2, 6, and 9, the expansion at a main surface of the substrate close to the printed board 8 is larger than the expansion at the other main surface. Thus, as shown in FIG. 12, the substrates warp upward in the direction away from the printed board 8. In other cases, the expansion at the main surface close to the printed board 8 is smaller than the expansion at the other main surface. In this case, as shown in FIG. 13, the substrates warp downward in the direction toward the printed board 8.